This invention relates to ion sensitive electrodes, and more particularly to electrochemical reference cells incorporating flushing means for removing contaminants or clogging material from the cells.
In many chemical processes, it is important to monitor or control the concentrations of selected ionic species in order to influence reaction by adjusting the concentrations thereof or modifying other reaction conditions. Typical of such measurements is the potentiometric determination of hydrogen ion concentration with pH probes incorporating a measuring cell and a reference cell. A common problem of electrochemical pH measurement or electrochemical determination of other ionic species is reaction, in the reference cell, between the heavy metal salts used as reference electrodes, such as silver chloride or mercury chloride, with certain ionic species present or generated in the aqueous medium in contact with the reference cell, such as reactants, by-products or impurities, soluble or insoluble, collectively hereinafter sometimes called "contaminants". Characteristically, a contaminant diffuses across an electrochemical junction (such as a porous, ion permeable member including ceramic plugs, membranes and fibrous materials) into the electrolyte solution of a cell and is carried by the electrolyte to the reference electrode where it reacts with the heavy metal salts. The resulting reaction products usually are insoluble and (together with the contaminant itself, if insoluble) collect at the junction. As the junction becomes clogged with the insoluble material, an erroneous potential is generated across the junction due to restriction of ion transport through the junction. Eventually the insoluble material will clog the junction and entirely prevent hydrogen ion determination or measurement of other ionic species.
In attempts to remedy the foregoing problem, double junction reference cells of many different designs have been developed, as illustrated by U.S. Pat. Nos. 4,282,081, 4,390,406 and 4,401,548. In these cells, a second porous junction is combined with the porous junction of the conventional reference electrode with the zone between the junctions filled with an electrolytic liquid such as a potassium chloride solution. Although this design reduces the rate of reference cell failures when the reference cells have only intermittent or brief exposures to a sample containing a contaminant, the failure rate greatly increases and becomes unacceptable when the reference cells are to be used for continuous, in-line measurements.
In another approach to alleviating the junction-clogging problem, reference cells have been designed with cleansing or flushing systems, frequently in combination with double junctions. U.S. Pat. Nos. 3,486,997, 3,997,420, 4,285,792 and 4,378,280 are representative of reference cells incorporating flushing means. Nevertheless, such cells remain unsatisfactory for various reasons, including requirements for awkward mechanical elements such as valves and pumps, limitation to flushing of only portions of the reference cells and inapplicability to continuous, long-term exposure to contaminants in a sample, (both quiescent liquids and mobile chemical process streams), wherein it is desired that the exposure will require only minimal care from plant personnel.